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The effects of pH, organic phosphates (2,3-diphospho- glycerate), and temperature on the functional properties of both adult and fetal hemoglobin Sassari alpha (Asp-126 --> His) have been studied. The functional properties of the adult variant are characterized by the following: (i) an oxygen affinity higher than that of normal HbA in all the experimental conditions used; (ii) a dramatic reduction of homotropic interactions (n(50) very close to unity); and (iii) a significant decrease of the effect of 2,3-diphosphoglycerate, which is 35% lower than that observed on HbA. The fetal variant shows an increased oxygen affinity compared with normal HbF and an almost abolished heme-heme interaction. The molecular basis of these functional differences is discussed in terms of the possible role played by the substitution of alpha (Asp-26 --> His) on the stability of the R state of the molecule due to a decreased interaction at the level of alpha(1) alpha(2) contact.